The invention described herein relates to a method and apparatus for manufacturing metals, and also relates to the metals so produced. In a particular aspect, the invented process is utilized for producing cobalt, and comprises the dissolution and purification of solutions of CoCl2 and/or CoSO4, followed by further refining and deposition by electrolysis. The electrolysis can be followed by vacuum melting to produce further refined cobalt. The cobalt produced is preferably xe2x80x9chigh-purityxe2x80x9d cobalt, with high-purity cobalt according to this invention being defined as having a total metallic purity of 99.99% (4N) or greater, excluding gaseous impurities. The high-purity cobalt produced is suitable for use in sputter targets and related microelectronic applications. The cobalt material can also be lower purity in cobalt, such as, for example, cobalt materials that are about 99.9% cobalt.
High-purity metals are desired for many modern processes, such as, for example, as solders, sputtering targets, and applications in semiconductor devices. For instance, high purity cobalt can be desired for formation of sputtering targets. In particular applications, a film of cobalt is sputter-deposited from a high-purity target, and onto a silicon substrate. The film is then subjected to a heat treatment to form cobalt disilicide (CoSi2). Cobalt disilicide has low resistivity and low formation temperature, and is considered a good alternative to titanium disilicide (TiSi2) in integrated circuit applications. It is thus possible that cobalt will partly replace titanium in the manufacture of new generation chips. Cobalt sputtering techniques can also be applied to the manufacture of data storage devices, flat panels and other similar products. Considering the rapid development of the electronics industry, it is believed that a potential market exists for cobalt targets of a purity of 4N or greater.
Cobalt is recovered as a co-product of copper in Central Africa, and as a by-product of hydrometallurgical refining of nickel elsewhere. In the African plants, copper-cobalt concentrates are roasted and leached in a sulfuric acid solution. Copper and cobalt are recovered separately from the leach solution by direct electrowinning. For hydrometallurgical refining of nickel, a variety of techniques such as selective precipitation and crystallization, solvent extraction and ion exchange, are used to separate cobalt from nickel. Cobalt is then electrowon from sulfate or chloride solutions. In addition to the electrowinning process, cobalt can also be produced as metal powder using a soluble cobaltic amine process. Nickel, as a sister element to cobalt, is always found in cobalt produced by these processes. Other impurities in the resulting cobalt include alkali metals (such as Na, K), radioactive elements (such as U, Th), transition metals (such as Ti, Cr, Cu, Fe) and gaseous impurities (with gaseous impurities being those measured by LECO, and being O, C, S, N, H).
Nickel is not easily removed from cobalt. This is because of the similarity of cobalt and nickel in a series of properties. Cobalt and nickel can form thermodynamically ideal liquid and solid solutions. The solidification of a Coxe2x80x94Ni system takes place in a temperature interval of only a few degrees. The standard electrode potentials of the reactions
Co2++2exe2x88x92xe2x86x92Co;
and
Ni2++2exe2x88x92xe2x86x92Ni
in aqueous solutions at 25xc2x0 C. are xe2x88x920.28V and xe2x88x920.23V, respectively. The difference of both potentials is only 0.05V. All of these factors make the separation of cobalt and nickel very difficult.
For the semiconductor industry, it can be important to minimize impurities in cobalt sputtering targets in order to prevent problems with semiconductor chips comprising sputter-deposited cobalt. Specifically, alkali metals (such as Na and K), non-metallics (such as S and C), and metallics (such as P within the context of this document) are undesirable because these elements are considered to be very mobile and may migrate from one semiconductor device layer to another. Fe is another element that can be undesirable. Specifically, Fe can affect the magnetic properties of a material, which causes concern for magnetic inconsistency. Further, Fe, as well as Ti, Cr, Cu can be undesirable in that they can cause problems with connections at semiconductor device interfaces. Additionally, gaseous impurities (such as oxygen) are undesirable since they can increase electrical resistivity of the cobalt and the cobalt silicide layer in semiconductor devices. Increasing 0 levels also increase particulates that form during application of metallization layers. These particulates can degrade or destroy a cobalt silicide layer. Ni impurities in cobalt are undesired since Ni can influence the pass-through flux of cobalt sputtering targets. And finally, radioactive elements such as U and Th are undesirable in Co since they emit alpha radiation, which can cause semiconductor device failures.
Other metals, besides cobalt, also have applications as high-purity materials (for instance as sputtering targets or as solders), and it would be desirable to develop purification methods which can be applied not only to cobalt, but also to other metals.
In accordance with the present invention there is provided a method and apparatus for producing high-purity metals. The invention also encompasses the high-purity metals which can be produced by the method and apparatus. In one aspect, the method is a combination of electrolysis and ion exchange followed by vacuum melting to produce cobalt of a desired purity. Specifically, a method of the present invention can comprise the following steps:
(a) Providing an electrolysis cell;
(b) Anodically dissolving cobalt metal into an electrolyte solution;
(c) Passing impure electrolyte solution at controlled pH and flow rate across a chelating ion exchange resin to remove contaminates and form a cleaned electrolyte solution; and
(d) Transferring the cleaned electrolyte solution to the cell and cathodically depositing purified metal at a cathode of the cell.
Methodology of the present invention can produce high-purity metal with minimum elemental impurities, and can be used, for example, in the formation of high-purity cobalt. The high-purity cobalt so produced is at least 99.99% cobalt, and in particular embodiments can comprise 99.9995% cobalt. The high purity cobalt can have total impurities (excluding gasses) of less than 100 ppm, and in particular embodiments can comprise total metallic impurities of less than 25 ppm, with total metallic impurities being defined as the sum of the elemental impurities Li, Be, B, Na, Mg, Al, Si, P, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Br, Rb, Sr, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, In Sn, Sb, Te, I, Cs, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg, Tl; Pb, Bi, Th, U, Cl and F (not including those at detection limits). It is noted that for purposes of interpreting this disclosure and the claims that follow, some elements are listed as xe2x80x9cmetallic impuritiesxe2x80x9d, even though the elements are not typically considered metals. Such elements are B, Si, P, As, Se, and Br.
Individual elemental impurities of cobalt produced in accordance with the present invention can be as follows: Na and K less than 0.5 ppm each, Fe less than 10 ppm (and in particular embodiments less than 8 ppm), Ni less than 5 ppm (and in particular embodiments less than 3 ppm), Cr less than 2 ppm (in particular embodiments less than 1 ppm, and in some embodiments less than 0.01 ppm), Ti less than 3 ppm (in particular embodiments less than 1 ppm, and in some embodiments less than 0.4 ppm), and O less than 450 ppm (and in particular embodiments less than 100 ppm). The method of chemical analysis used to determine the metallic impurities set forth herein is glow discharge mass spectroscopy (GDMS) and the method used to determine gaseous impurities is LECO, unless otherwise specified.